1. Field of the Invention
The present invention relates to an electronic display (electro optical device) formed by fabricating an EL (electro luminescence) on a substrate. In particular, the present invention relates to a display device using a semiconductor element (an element which uses a semiconductor thin film). Further, the present invention relates to an electronic device using an EL display in a display portion and the method of detecting the EL display.
The EL element herein refers to both an element that utilizes light emission from a singlet exciton (fluorescence) and an element that utilizes light emission from a triplet exciton (phosphorescence).
2. Description of the Related Art
Recently, a technique for forming a thin film transistor (hereinafter, referred to as TFT) on a substrate has been remarkably developed, and a development of its application to an active matrix display device has been continuously made. In particular, TFTs using a polysilicon film can operate at high speed, because such TFTs have a higher field effect mobility than TFTs using a conventional amorphous silicon film. Therefore, the control of pixels, which has been conventionally conducted by a driver circuit provided outside a substrate, can be performed by a driver circuit provided on the same substrate on which the pixels are provided.
Such an active matrix display device includes various circuits and elements formed on the same substrate. With this structure, the active matrix display device provides various advantages such as reduced manufacturing cost, reduced size of a display device, an increased yield, and a reduced throughput.
Furthermore, an active matrix EL display device including an EL element as a self-luminescent element has been actively studied. The EL display device is also called Organic EL Display (OELD) or Organic Light Emitting Diode (OLED).
In contrast with the liquid crystal display device, the EL display device is self-luminescent. The EL element has such a structure that an EL layer is sandwiched between a pair of electrodes (anode and cathode). However, the EL layer has normally a lamination structure. As a typical example of the lamination structures, a lamination structure “hole transport layer/light emitting layer/electron transport layer” proposed by Tang et al. of Eastman Kodak Company is cited. This structure has an extremely high light emitting efficiency. For this advantage, most light emitting devices, which are currently under study and development, employ this structure.
Furthermore, the light emitting device may have such a lamination structure that a hole injection layer, a hole transport layer, a light emitting layer and an electron transport layer are deposited on an anode or a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, and an electron injection layer are deposited on an anode in this order. Moreover, the light emitting layer may be doped with a fluorescent pigment or the like.
All layers formed between a cathode and an anode are referred to generically as EL layers within this specification. The above stated hole injecting layer, hole transporting layer, light emitting layer, electron transporting layer, electron injecting layer, and the like are therefore all contained within the EL layer.
A predetermined voltage is then applied to the EL layer having the above structure by a pair of electrodes, thus recombination of a carrier thus occurs in the light emitting layer, and light is emitted. Note that the emission of light by the EL element is referred to as driving the EL element throughout this specification. Further, an EL element formed by an anode, an EL layer, and a cathode is referred to as an EL element throughout this specification.
As a method of driving an EL display device, an analog driving method (analog drive) can be given. The analog drive of an EL display device is described with reference to FIGS. 10 and 11.
FIG. 10 shows a structure of a pixel portion of an EL display device that is driven in an analog manner. Gate signal lines (G1 through Gy) to which a gate select signal from a gate signal line driver circuit is input are connected to a gate electrode of a switching TFT 1801 included in each pixel. One of a source region and a drain region of the switching TFT 1801 included in each pixel is connected to source signal lines (also referred to as data signal lines) S1 to Sx to which an analog video signal is input, whereas the other is connected to a gate electrode of an EL driver TFT 1804 included in each pixel and a capacitor 1808 included in each pixel.
A source region and a drain region of the driver TFT 1804 included in each pixel are connected to power source supply lines V1 through Vx and to an EL element 1806, respectively. An electric potential of the power source supply lines V1 through Vx is referred to as an power source electric potential. The power source supply lines V1 through Vx are connected to the capacitors 1808 included in the respective pixels.
The EL element 1806 includes an anode, a cathode and an EL layer sandwiched between the anode and the cathode. If the anode of the EL element 1806 is connected to the source or the drain region of the driver TFT 1804, the anode and the cathode of the EL element 1806 become a pixel electrode and an opposing electrode, respectively. On the other hand, if the cathode of the EL element 1806 is connected to the source or the drain region of the driver TFT 1804, the anode and the cathode of the EL element 1806 become an opposing electrode and a pixel electrode, respectively.
Note that the electric potential of the opposing electrode is referred to as an opposing electric potential in this specification. Note also that an power source for imparting the opposing electric potential to the opposing electrode is referred to as an opposing electric power supply. The electric potential difference between the electric potential of the pixel electrode and the electric potential of the opposing electrode is an EL driver voltage, and the EL driver voltage is applied to the EL layer.
FIG. 11 shows a timing chart in the case where the EL display device shown in FIG. 10 is driven in an analog manner. The period from the selection of one gate signal line until the selection of a next gate signal line is called one line period (L). The period from the display of one image to another image corresponds to one frame period (F). In the case of the EL display device shown in FIG. 10, since there are y gate signal lines, y line periods (L1 to Ly) are provided within one frame period.
With the enhancement in resolution, the number of line periods within one frame period increases. As a result, the driver circuit must be driven at a high frequency.
An power source electric potential at the power source supply lines (V1 through Vx) is held constant, and an opposing electric potential at the opposing electrodes is also held constant. The opposing electric potential has a potential difference with the power source electric potential to such a degree that a EL element 1806 emits light.
The gate signal line G1 is selected in the first line period L1 by a gate signal input to the gate signal line G1 from the gate signal line driver circuit. Then an analog video signal is then input in order to the source signal lines S1 to Sx. All of the switching TFTs 1801 connected to the gate signal line G1 are in an ON state, and therefore the analog video signal input to the source signal lines S1 to Sx is input to gate electrodes of the driver TFTs 1804 through the switching TFTs 1801.
The description here takes as an example a timing chart of the case where the switching TFT 1801 and the driving TFT 1804 are both n-channel TFTs. The switching TFT and the driving TFT may instead be p-channel TFTs, or one of them may be an n-channel TFT while the other is a p-channel TFT.
In this specification, the TFT being turned ON means that the gate voltage of the TFT is changed such that the source-drain thereof is brought into conductive state.
The amount of a current flowing through a channel formation region of the driver TFT 1804 is controlled by a level of an electric potential (voltage) of a signal input to the gate electrode of the driver TFT 1804. Accordingly, the electric potential applied to the pixel electrode of the EL element 1806 is determined by the level of the electric potential of the analog video signals input to the gate electrode of the driver TFT 1804. Then, the EL element 1806 is controlled by the electric potential of the analog video signals to emit light.
When the above-described operation is repeated to complete the input of analog video signals to the source signal lines (S1 through Sx), the first line period (L1) terminates. One line period may alternatively be constituted by the period until the completion of input of the analog video signals to the source signal lines (S1 through Sx) and a horizontal blanking period. Then, a second line period (L2) starts where a gate signal line G2 is selected by a gate signal. And, as in the first line period (L1), analog video signals are sequentially input to the source signal lines (S1 through Sx) during the second line period.
When all gate signal lines (G1 through Gy) are selected in this manner, all lines periods (L1 through Ly) are completed. The completion of all the line periods (L1 through Ly) corresponds to the completion of one frame period. All pixels perform display during one frame period to form an image. One frame period may be alternatively constituted by all line periods (L1 through Ly) and a vertical blanking period.
The amount of light emitted by the EL element 1806 is thus controlled in accordance with the analog video signal, and gray scale display is performed by controlling the amount of light emitted. This method is namely a driving method referred to as an analog driving method, gray scale display is performed by changing the electric potential of the analog video signal input to the source signal lines.
In the conventional EL display device, the drain region of the driving TFT 1804 in the pixel portion is connected only to the EL element 1806 as shown in FIG. 10.
TFTs are formed on a substrate having an insulating surface in order to constitute pixel TFTs (each formed of a switching TFT and a driving TFT) and driver circuits (including a source signal line driving circuit and a gate signal line driving circuit). An EL material is then deposited and the driving TFT is electrically connected to an EL element. The manufacturing steps prior to the step of depositing the EL material are called herein TFT steps.
Before the EL material is deposited, the drain region of the driving TFT in the conventional display device is thus in an open state from the design of the circuit. Whether a certain pixel TFT operates normally or not cannot be judged until the EL material is deposited to complete the display device and lighting test is performed on the completed device. Therefore, it is not until after the manufacturing process reaches the final step that a display device incapable of normal display because of a defective pixel TFT can be found out. This is utterly a waste.
As described above, the conventional EL display device does not allow its pixel TFTs to be checked for their operation during the steps prior to deposition of the EL material, thereby incurring a waste in manufacturing cost.